Our invention relates to an apparatus for translating an electronically produced optical image into an electric signal, and more specifically to an system comprising a cathode-ray tube (CRT) or the like for electronically displaying electric waveforms or other images on a screen, and fiber optics means for transmitting the image from the screen to an image sensor by which the image is converted from optical into electrical form.
It has been known to employ a CRT in combination with a fiber optics faceplate for photorecording the CRT traces. Comprised of an array of a multiplicity of short optical fibers, the fiber optics faceplate is held against the screen of the CRT for transmitting the optical image on to a photosensitive film with a minimum of loss. This known system makes possible the recording of ultrahigh speed transients. However, the evaluation and analysis of the recordings on the film has often been no easy task.
An optical lens system has also been used in combination with a CRT in substitution for the fiber optics faceplate. Placed in front of the CRT screen, the lens system focuses the screen image on an image sensor such as charge-coupled devices or photodiodes thereby to be translated into electric information. We object to the use of the optical lens system for transmission of the optical data. It involves greater transmission loss than does the fiber optics faceplate as the total amount of light transmitted is limited by the solid angle which is determined by the screen size, the lens diameter and the distance between screen and lens system. The optical lens system also tends to cause pattern distortions, making it difficult to accurately transmit the screen image to the image sensor.
The familiar image intensifier represents yet another conventional approach to the recording, or the translation into electric signals, of optical images. Typically, the image intensifier has a fiber optics plate with a photocathode attached to its beam-exit side for converting the incoming optical information into a beam of electrons. The beam of electrons travels through a microchannel plate thereby to be intensified. The intensified electron beam falls on a screen to visualize the input optical information. The image intensifier has another fiber optics plate held against the beam-exit side of the screen for transmitting the visualized light to an image sensor for either recording it or converting it into electric form.
The image intensifier has its own drawback well known to those versed in the optoelectronics art. The image intensifier has an anode on the beam-entrance-side face of the beam-exit-side fiber optics plate. The magnitude of the voltage applied to this anode determines the luminance of the screen. However, when this voltage exceeds 10 kilovolts, a multiplicity of starlike dots appear throughout the output from the image sensor, ruining the quality of the image and, in the worst case, making the image sensor totally inoperative. A voltage of only several kilovolts has therefore been impressed to the anode in order to avoid the appearance of such bright dots. The microchannel plate has been a conventional expedient designed to compensate for the low anode voltage. However, the very construction of the microchannel plate makes it very expensive, adding substantially to the total cost of the image intensifier.
As far as we know, the following explanation is most widely accepted by the specialists as a theory accounting for the appearance of the undesired dots. In the fabrication of glass fibers for use in the fiber optics plates of the image intensifier, there are supposedly unavoidably created localized parts of abnormally low electrical insulation resistance. The image sensor at the output stage of the image intensifier is subject to positive and negative voltages of the order of tens of volts in absolute value. Therefore, upon application of a positive voltage of 10 kilovolts or more to the anode, leakage current flows across the beam-exit-side fiber optics plate through its parts of low insulation resistance, resulting in the electrification of parts of the beam-exit-side face of that fiber optics plate. The accumulated charges on liberation from the fiber optics plate are transferred to the image sensor being held in contact with, or close to, the fiber optics plate. Another explanation is that the localized charges on the fiber optics plate induce charges on the image sensor.
We also object to the conventional image intensifier because of the considerable transmission loss taking place particularly when the image sensor is positioned opposite the fiber optics plate with a spacing therebetween. The transmission loss is due to the phenomenon known as Fresnel reflection between the confronting faces of the fiber optics plate and the image sensor. Normally, air intervenes between these opposed faces when they are spaced from each other. The refractive index of the intervening medium in this case is approximately 1.0 whereas the refractive index of the fiber optics plate is from 1.6 to 1.8, and that of the beam-entrance face of the image sensor is 2.0. These great differences in reflective index are a cause that must be remedied for reduction of the transmission loss.
A similar problem has also occurred, though to a lesser degree, even when the fiber optics plate and the image sensor are positioned in contact with each other. This is because their contacting faces can never be perfectly flat.